THE VASCULAR CRYPTOGAMS 89 



Fig. 40, things are simplified. Here there is little need 

 for mechanical strength, as the weight of the branch is 

 trifling, and so we find the whole cortex utilised for 



O' 



assimilation ; the vallecular spaces also are absent. 

 Functionally these little twigs do duty as leaves. 



The epidermis is chiefly remarkable for its strongly 

 silicified outer cell-walls, which make the surface 

 extremely hard. If all the organic matter be completely 

 burnt away, a perfect skeleton of silex, still showing every 

 marking on the cell-walls, is left behind. The stomata are 

 peculiar, because the guard-cells are. completely covered in 

 on the top by the subsidiary cells, so that a double pair 

 "of guard-cells, one above the other, seems to be present. 



The description of the structure of the stem, which 

 we have just given, refers more especially to the sterile 

 shoots growing above ground. Both the underground 

 rhizomes and the fertile shoots are somewhat modified in 

 structure. In the former the epidermis is destitute of 

 stomata, and the cortex of chlorophyll-tissue, while 

 mechanical tissues are little needed and little developed ; 

 thus the whole differentiation of the outer tissues is much 

 reduced. 



In E. arvense the pith of the rhizome is solid, and this 

 is often the case in the smaller aerial branches also, as 

 shown in our Fig. 39. The tubers consist simply of 

 parenchyma crowded with starch, and traversed by a 

 few reduced vascular bundles ; each tuber corresponds to 

 a single internode. 



The fertile stem being a transitory organ, with no 

 other function than to bear the cone, has a simplified 

 structure, and is destitute at once of stomata, chlorophyll, 

 and sclerenchyma. Throughout all parts of the shoot, 

 however, the vascular system maintains the same struc^ 



